WHAT SOILS ARE BEST FOR AGRICULTURE? 237 



coarse one. The layers of sediment, too, may vary in 

 composition, depending upon the kind of rock from which 

 they are made. We know that sandstone forms sandy 

 soils, but we may not know that clayey soils come from 

 the breaking down of shales and feldspars. Igneous 

 and metamorphic rocks may yield both clay and sand. 

 Sand by itself makes a barren soil because there is not much 

 in it except glass-like silica or quartz. Plants need a 

 large variety of elements. Limestone produces a lime- 

 stone soil which is usually very fertile. The soils from 

 feldspar furnish potassium, sodium, calcium, magnesium, 

 and iron, making a rich soil. But if it makes a compact 

 clay, it is too wet and lacks air for good crops. Mixed 

 with sand, it makes a good soil for crops. Soils, there- 

 fore, differ in different parts of the country, depending 

 upon the rocks found there, or transported there. The 

 lower Mississippi region is very fertile because of the 

 rich soil brought down in floods and deposited where 

 the river overflows its banks. 



Kinds of Soils. Those of us who have gardens know 

 that the fertility of our plot depends largely on the soil 

 which makes it up. Most soils may be divided into the 

 following general groups : Gravel, composed of a mixture 

 of coarse sand and pebbles ; sand, largely made of quartz, 

 produced from granite or sandstone ; clay, rock ground 

 up so fine that it is not gritty when rubbed between the 

 fingers, feels sticky, molds rather easily with water, and 

 becomes hard when dry; silt, particles too fine to class 

 as sand and too coarse to be a clay ; loam, a combination 

 50 per cent of sand and 50 per cent clay and silt together ; 

 humus, largely decayed plant and animal matter. For 

 gardens the latter material is considered quite necessary. 

 Demonstration 2. Water in Soil. 



Materials. Four student-lamp chimneys ; equal volumes of 

 dry sand, clay, loam, and humus. Tie two or three thicknesses of 



